Turbochargers are a type of forced induction system. They deliver air to the engine intake at a greater density than would be possible in a typical aspirated engine configuration. As a result, more fuel can be combusted, which, in turn, can boost the engine's horsepower without significantly increasing engine weight. FIG. 1 shows a prior art system in which a turbocharger (10) includes a turbine wheel (12), a compressor wheel (14), and a connecting shaft (16). The turbine wheel (12) is located within a turbine housing (18), and the compressor wheel (14) is located in a compressor cover (20). The turbine wheel (12) is driven by exhaust gas exiting an internal combustion engine. The rotation of the turbine wheel (12) is communicated to the compressor wheel (14) by the shaft (16). The compressor wheel (14) is used to increase the pressure of intake air prior to mixing with fuel and combustion in the engine.
The shaft (16) extends through a bearing housing (22) and is mounted for rotation in bearings, including journal bearings (24). The speeds at which the shaft (16), turbine wheel (12) and compressor wheel (14) are rotated are very high, and can be in excess of 250,000 rpm. Therefore, the bearings used to support the shaft (16) must be lubricated with pressurized oil. However, the oil in the bearing system is susceptible to breakdown and coking if operating temperatures become too extreme. Further, seal ring(s), such as piston rings 26 (see FIG. 2), are used to restrict the flow of gases and oil from within the bearing housing (22) to the turbine housing (18).
The exhaust gas from the engine is typically at temperatures ranging from 740° C. to 1050° C., depending upon the fuel used. Because of these high temperatures, turbochargers typically use a turbine heat shield (28) which is positioned between the turbine wheel (12) and the bearing housing (22). The heat shield (28) protects the bearing housing (22), the parts within the bearing housing (22), and the compressor stage, from unwanted transfer of thermal energy from the exhaust gas in the turbine housing (18). More particularly, one function of the turbine heat shield (12) is to impede the conductive and radiative flow of heat from the exhaust gas, through the bearing housing (22), to the compressor stage, the efficiency of which can be adversely affected by increases in the temperature of the compressor cover (20). Another function of the turbine heat shield (28) is to impede the flow of heat from the exhaust gas to the piston ring(s) (26) and journal bearings (24).
The heat shield (28) is a separate component that is made of a different material that the bearing housing (22). The material of the heat shield (28), typically an austenitic stainless steel (e.g., SS-321 or SS-348), has a lower coefficient of thermal conductivity than the material of the bearing housing (22), typically cast gray iron or ductile iron. As a result, a stainless steel heat shield (28) can provide more impedance to the conductive transmittal of heat than would be the case if the turbine heat shield (28) was fabricated out of the same cast iron as that used in the bearing housing (22). Thus, providing a separate heat shield made of a different material provides heat transfer advantages.
However, there are many difficulties with the inclusion of a separate heat shield (28) into the turbocharger (10). For instance, the heat shield (28) must be adequately retained so that it is not loose in which it could rattle against the bearing housing (22) or turbine housing (18), or touch the turbine wheel (12) or the shaft (16) and turbine wheel (12), both of which are rotating at very high speed. To that end, the heat shield (28) is typically attached to the bearing housing (22). One common way of attaching the heat shield (28) to the bearing housing (22) is by staking the stainless steel heat shield (28) to the bearing housing (22). In such case, a portion of the bearing housing (22) is deformed over a flange (30) of the heat shield (28). However, the staking procedure is difficult and results in deformation of a portion of the bearing housing (22).
In some turbochargers (e.g., non-VTG turbochargers), the heat shield is retained by sandwiching a portion of the heat shield (e.g. the flange) between the turbine housing and the bearing housing. However, it can be difficult to maintain the clamping load on the heat shield during turbocharger operation. A loss of clamp load can result in gas and soot leakage from the turbocharger to the engine environment. In either manner of attachment (staking or clamping), the concentricity of the bore in the heat shield to the bore of the journal bearings in the bearing housing is not easy to achieve due to the clearances and tolerances required for the assembly of the separate pieces (turbine housing, heat shield, bearing housing) in practice.
Further, the use of a separate heat shield can cause difficulties at the assembly/processing stage of the core assembly (the assembly of the rotating assembly in the supporting bearing housing). During this stage, a core balance procedure is typically performed in which the rotating assembly is spun up to medium-to-high speed, and the balance of the rotating assembly is checked and adjusted where necessary. However, the turbine heat shield is often loose (that is, not fully retained) until it is trapped between the bearing housing and turbine housing at the next step in assembly. As a result, problems can arise at the core balance station. Thus, the heat shield must somehow be kept from touching the rotating assembly during this procedure. This is often done by clipping the heat shield to another structure so it is not hanging freely, or by clamping it in place as part of the high speed core balance procedure. However, such steps are time consuming, challenging and not consistently repeatable.
Thus, there is a need for a heat shield configuration and related system that can minimize one or more of the above concerns.